When an electronic component such as a semiconductor chip is mounted on a wiring substrate, the outermost layer of the wiring substrate is formed from an insulation material that is referred to as a solder resist. The solder resist protects the wiring substrate from solder, contamination, and the like. The solder resist includes openings that expose a wiring layer, which is located below the solder resist, as electronic component mount pads. The electronic component mount pads are connected to the electronic component. Japanese Laid-Open Patent Publication Nos. 2011-258590 and 2008-258373 each describe an example of the wiring substrate.
FIG. 17 is a partially enlarged cross-sectional view of a wiring substrate in the related art. A wiring layer 101 is formed on an upper surface of an insulation layer 100, and a solder resist layer 102 is formed on the upper surface of the insulation layer 100 to cover the wiring layer 101. The solder resist layer 102 includes through holes 102X, which partially expose the wiring layer 101 as electronic component mount pads 101P. The through holes 102X are formed, for example, through photolithography. Each pad 101P includes, for example, a solder bump used for connection with an electronic component. The solder bump is formed by mounting a solder ball on the pad 101P and melting the solder ball. In the solder resist layer 102, each through hole 102X is defined by a curved wall surface to improve the mounting of the solder ball.
Sophistication of recent semiconductor chip has increased the number of connection terminals (pins) arranged in a semiconductor chip and narrowed the pitch of the connection terminals. Thus, there is demand for a wiring substrate that includes pads arranged at a narrow pitch (e.g., a pitch of 50 μm or less). However, the curved wall surface of the through hole 102X increases the bore diameter Φ10 of the through hole 102X. Consequently, the through holes 102X impose limitations on the arrangement of the pads 101P. Thus, it is difficult to narrow the pitch of the pads 101P.